Mac OS X's GUI may have its flaws, the OS may still crash once in a while, and there are still a few straggling applications we're waiting for.
But the one reason that Mac OS X is the best thing that has happened to the Mac community in a long time is Darwin. In short, it's an open-source UNIX variant based on FreeBSD, but you can read more about that anywhere.
The thing is, UNIX means legitimacy to a lot of people. Mac OS X is becoming a hit on Slashdot because it's a user-friendly UNIX, whereas before Mac OS X, people would never give the Macintosh a second thought.
I personally have learned more about UNIX in the last one and a half years than I ever thought I would know about it, because had I not had it all right there in front of me, I would probably never have bothered. UN*Xes, in my experience, generally have a pretty steep learning curve, and I wonder how many people have taught themselves the fundamentals of UNIX on an OS X box just because once you have OS X, all you need is a bit of inquisitiveness to get started.
I was always happy to be a Mac user but it's only since the advent of OS X that I can kinda be proud to be one too.